Since carbon fibers are excellent in strength and elastic modulus though light in weight, composite materials obtained by combining carbon fibers with various matrix resins are used in many fields including aircraft members, spacecraft members, motor vehicle members, seacraft members, civil engineering and architectural materials and sports articles. In the composite materials obtained by using carbon fibers, the adhesion between the carbon fibers and the matrix resin is important in order that the excellent properties of the carbon fibers can be used.
In order to enhance the adhesion between the carbon fibers and the matrix resin, normally oxidation treatment such as gas-phase oxidation or liquid-phase oxidation is applied to the carbon fibers as a method for introducing oxygen-containing functional groups into the surface of the carbon fibers. For example, a method of enhancing the interlaminar shear strength used as an indicator of adhesion by electrolytically treating carbon fibers is proposed (see patent document 1). However, in recent years, as the level of properties required for the composite materials rises, the adhesion that can be achieved by such oxidation treatment alone becomes less sufficient.
On the other hand, carbon fibers are fragile and poor in bundling properties and abrasion resistance, fuzz and fiber breakage are likely to occur. For this reason, normally, a method of coating carbon fibers with a sizing agent is used.
For example, methods of coating carbon fibers with bisphenol A diglycidyl ether as a sizing agent are proposed (see patent documents 2 and 3). Further, methods of coating carbon fibers with a polyalkylene oxide addition product of bisphenol A as a sizing agent are proposed (see patent documents 4 and 5). Furthermore, methods of coating carbon fibers with a material obtained by adding epoxy groups to a polyalkylene oxide addition product of bisphenol A as a sizing agent are proposed (see patent documents 6 and 7). Moreover, methods of coating carbon fibers with an epoxy addition product of a polyalkylene glycol as a sizing agent are proposed (see patent documents 8, 9 and 10).
In addition, a method of coating carbon fibers with an urethane compound having an epoxy group and a quaternary ammonium salt as a sizing agent is proposed (see patent document 11). Either with the proposed method, the adhesion between the carbon fibers and the matrix resin cannot be enhanced, though bundling properties and abrasion resistance can be enhanced.
These methods are known to enhance the bundling properties and abrasion resistance of carbon fibers. However, these conventional proposals lack the technical idea of positively enhancing the adhesion between the carbon fibers and the matrix resin by using a sizing agent, and actually cannot highly enhance the adhesion between the carbon fibers and the matrix resin.
On the other hand, methods of coating carbon fibers with a specific sizing agent for the purpose of enhancing the impregnability of the matrix resin into the carbon fibers are used.
For example, a method of coating carbon fibers with a cationic surfactant having a surface tension of 40 mN/m or lower and a viscosity of 200 mPa·s or lower at 80° C. as a sizing agent is proposed (see patent document 12). Further, a method of coating carbon fibers with an epoxy resin, water soluble polyurethane resin and a polyether resin as sizing agents is proposed (see patent document 13). These methods are found to enhance the bundling properties of the carbon fibers and the impregnability of the matrix resin into the carbon fibers. However, these conventional proposals also lack the technical idea of positively enhancing the adhesion between the carbon fibers and the matrix resin by using a sizing agent, and actually cannot highly enhance the adhesion between the carbon fibers and the matrix resin.
As described above, sizing agents are hitherto used as so-called sizes for the purpose of enhancing processability or for the purpose of enhancing the impregnability of the matrix resin into the carbon fibers, and few studies have been made to enhance the adhesion between the carbon fibers and the matrix resin by using a sizing agent. Further, even in the studies made, the obtained effect is limited such that the effect of enhancing the adhesion is insufficient or that the effect can be exhibited only in the case where special carbon fibers are used in combination.
For example, a method of coating carbon fibers with N,N,N′,N′-tetraglycidyl metaxylylenediamine as a sizing agent is proposed (see patent document 14). However, though it is demonstrated that this proposed method enhances the interlaminar shear strength used as an indicator of the adhesion compared with the case of using bisphenol A glycidyl ether, the effect of enhancing the adhesion is still insufficient. Further, the N,N,N′,N′-tetraglycidyl metaxylylenediamine used in this proposal contains an aliphatic tertiary amine in the structure thereof and is nucleophilic, and therefore self-polymerization reaction occurs. As a result, the carbon fiber bundles become harder with the lapse of time and there is a problem that the processability declines.
Further, a method of coating carbon fibers with a mixture comprising a vinyl compound monomer having a glycidyl group and an amine curing agent for an epoxy resin as a sizing agent is proposed (see patent document 15). However, though this proposed method is demonstrated to enhance the interlaminar shear strength used as an indicator of the adhesion, compared with a case where no amine curing agent is used, the effect of enhancing the adhesion is still insufficient. Further, there is a problem that in the step of drying the sizing agent, the glycidyl groups and the amine curing agent react and it become a high molecular weight, that as a result, the carbon fiber bundles become so hard as to lower the processability, and furthermore that the gaps among the carbon fibers become so narrow that the impregnability of the resin declines. Another method of using an epoxy-based compound and an amine curing agent together as a sizing agent is also proposed (see patent document 16). However, according to this proposal, while the handling properties and impregnability of fiber bundles are enhanced, the sizing agent enhanced in molecular weight on the surface of carbon fibers may form a film, to inhibit the adhesion between the carbon fibers and the epoxy matrix resin as the case may be.
Moreover, a method of coating carbon fibers with an amine compound is proposed (see patent document 17). However, though this proposed method demonstrates that the interlaminar shear strength used as an indicator of the adhesion can be enhanced compared with the case of no coating, the effect of enhancing the adhesion is still insufficient. This proposal does not describe the detail of the mechanism of enhancing the adhesion, but the mechanism is estimated approximately as described below. In this proposal, diethylenetriamine and xylenediamine respectively containing a primary amino group, and piperidine and imidazole respectively containing a secondary amino group are used as amine compounds. Since any of the amine compounds contains active hydrogen in the molecule, it is considered that the active hydrogen acts on the epoxy matrix resin, to promote the curing reaction, and that, for example, the hydroxyl groups produced by the reaction between the epoxy matrix and the aforementioned amine compound and the carboxyl groups, hydroxyl groups and the like on the surface of carbon fibers form hydrogen-bondable interactions, to enhance the adhesion. However, as described before, the result of enhancing the adhesion by this proposal is still insufficient, and does not satisfy the requirement for the composite materials of recent years.
As a further other example of using an amine compound as a sizing agent, a method of using a hardened product comprising a thermosetting resin and an amine compound is proposed (see patent document 18). In this proposal, as the amine compound, m-xylenediamine containing a primary amino group, piperazine containing a secondary amino group or the like is used. The main purpose of this proposal is to positively react the active hydrogen contained in the amine compound and a thermosetting resin typified by an epoxy resin, for obtaining a hardened product, thereby enhancing the bundling properties and handling properties of carbon fiber bundles. The carbon fiber bundles are limited for use as chopped fibers, and the mechanical properties concerning the adhesion of molded articles after melt kneading with a thermoplastic resin are still insufficient.
Further, a method of using carbon fibers having a surface oxygen concentration (O/C), surface hydroxyl group concentration and carboxyl group concentration respectively in specific ranges as carbon fibers, and coating the carbon fibers with an aliphatic compound having a plurality of epoxy groups used as a sizing agent is proposed (see patent document 19). However, though the proposed method demonstrates that EDS as an indicator of the adhesion can be enhanced, the effect of enhancing the adhesion between the carbon fibers and the matrix resin is still insufficient. Further, the effect of enhancing the adhesion can be exhibited only in the limited case of using specific carbon fibers in combination.